The possible reasons for the red patch in the armpit are as follows: 1. In most cases, the area has an eczema-like reaction, or local allergic dermatitis due to allergies, especially contact dermatitis, due to contact with allergic substances resulting in local inflammatory erythema, this erythema is mainly due to contact with allergic substances after the local skin metabolic changes, if the allergic factors lead to, need to actively carry out If it is caused by allergic factors, anti-allergic treatment is needed; 2. Infectious factors, such as the presence of patchy erythema with clear borders due to Trichophyton rubrum infection, sometimes without obvious itching symptoms. In this case, it is clinically necessary to test for fungal infection, and if the test is positive, it is determined to be a Trichophyton rubrum infection.